This invention relates to a horizontal output circuit for a cathode ray tube, especially to a horizontal output circuit which is capable of correcting picture distortion caused by a change of load voltage at the anode of the CRT.
A horizontal output circuit including a series circuit of a horizontal deflection coil and S-shape capacitor connected in parallel with the horizontal output transistor, and in which an additional capacitor is connected in series to the S-shape capacitor and across the output of a further switching transistor, has been invented by the same inventor as the present invention and is disclosed in Japanese Laid-Open Patent No. 160378/1984. In this circuit, the further capacitor is connected between the series circuit, including the horizontal deflection coil and the S-shaped capacitor, and ground. As a result, picture distortion in which the horizontal length of the picture on the screen of the CRT is increased due to a decrease of the high voltage at the anode of the picture tube, is corrected by compensating the amplitude of the horizontal deflection current.
More particularly, in this prior circuit the voltage across the S-shape capacitor is reduced by the further capacitor interposed in series into the charge path for the S-shape capacitor during a time when the further switching transistor connected to the further capacitor is in the OFF state. Thus, a more stable picture in the face of fluctuations in the level of the high voltage is possible.
However, it has been found that in such a circuit, the regulation characteristic of the high voltage is deteriorated by the decrease in the amplitude of the horizontal flyback pulse resulting from the decrease of the amplitude of the horizontal deflection current caused by the decrease of the voltage across the S-shape capacitor. This deterioration of the regulation characteristic of the high voltage produces (1) a lack of brightness in the picture, (2) a decrease of the low DC voltage used for other circuits, such as the sound amplifier, obtained by rectifying and smoothing the low amplitude flyback pulse generated at another winding of the flyback transformer, (3) a decrease of the focus voltage generated at an intermediate terminal of the secondary winding of the flyback transformer, and so on. Thus, it has been found that although the prior circuit solves one problem, it also creates other problems at the same time.